## AQA GCSE Half Life

**Half Life**

There are two possible definitions, either is suitable:

**The half-life of a radioactive isotope is the time it takes for the ****number of nuclei of the isotope in a sample to halve. **

**The time it takes for the count rate (or activity) from a sample containing the ****isotope to fall to half its initial level.**

The one you would choose, depends on the situation. If the the data is showing the number of nuclei decreasing, then use the first one. If the data shows the count rate decreasing, then the 2nd one would be used.

**Half life graphs**

At the start the first activity reading is 10 x 10^{6} Bq.

To find the half life, we will halve this value which is 5 x 10^{6} Bq. Then draw a horizontal line from axis at 5 x 10^{6} Bq to graph line. Finally, continue to draw the line, but downwards toward the time axis.

**The line is shown in red, the half life for this isotope is 4 minutes. **

Sometimes, you need to show constant half life on a graph, so the process needs to be repeated. As shown in the graph below.

The graph above shows two half lives. Each half life is 4 minutes.

1st half life starts at 0 minutes and ends at 4 minutes.

2nd half life starts at 4 minutes and ends at 8 minutes.

**This shows us that each isotope has a constant half life. **

This means that for this isotope, every 4 mintes the count rate will halve.

**Tip: In the exam, draw the lines onto the graph with a ruler, there is often a mark for this!**

**Practice Questions**

1.Using the graph below estimate the half life for the sample.

2. Using the graph below estimate the half life for the sample

3. Use the data in the table below to plot your own graph and calculate the half life.

Time (seconds) | Activity (counts per second) |
---|---|

0 | 241 |

30 | 212 |

60 | 188 |

90 | 165 |

120 | 145 |

150 | 127 |

180 | 112 |

210 | 99 |

240 | 87 |

270 | 76 |

300 | 67 |

330 | 59 |

Absorption and Emission of EM Radiation

JJ Thomson and Plum pudding model

Ernest Rutherford and the Nuclear Model

Niels Bohr changing the Nuclear Model

Discovering the Proton and Neutron

Measuring radiation from radioactivity

Radiation types and properties

Random nature of radioactive decay

Radioactive contamination or irradiation

Hazards of contamination and irradiation

Studies on the effects of radiation on humans

Different half lives of radioactive isotopes

Nuclear Fission Chain Reaction

Writing nuclear fission equations